Geothermal heating involves the use of geothermal energy for heating applications. An increasingly popular manner of taking advantage of geothermal energy stored in the ground is by means of borehole heat exchangers (BHE). A BHE comprises one or more pipes installed in a borehole in the ground. A BHE generally has a pipe for heat transfer fluid flow downwards in the borehole and a pipe for heat transfer fluid flow upwards in the borehole. By means of a pump, to which the BHE may be connected, the heat transfer fluid can circulate in the BHE in order to continually absorb heat from the ground or to store heat in the ground.
One type of BHE is a U-pipe installation, which comprises two equal cylindrical pipes connected at the bottom of the borehole, allowing a heat transfer fluid to flow downwards in one of the pipes and upwards in the other in order to exchange heat with the surrounding ground.
Another type of BHE is the coaxial BHE, which has been found to have superior heat transfer efficiency compared to U-pipe installations. Coaxial BHEs are for example disclosed in the paper “First experiences with coaxial borehole heat exchangers” by J. Acuña et al, IIR Conference on Sources/Sinks alternative to the outside Air for HPs and AC techniques, Padua 2011. In section 3 of this paper, a pipe-in-pipe design is disclosed where the coaxial BHE comprises an energy capsule which consists of a thin hose, and a central pipe provided with insulation and arranged in the energy capsule. Water can thereby flow in one direction in the annular space between the energy capsule and in the opposite direction in the central pipe for exchanging heat with the surrounding ground.
However, both the production and installation of such a pipe may be unnecessarily complicated, and the robustness of this design may be lower than what would be ideal. In view of the above, there is a desire to improve existing coaxial BHEs.